1. Field of the Invention
The present invention generally relates to an improvement of a cathode ray tube, and particularly to a cathode ray tube of high resolution.
2. Description of the Prior Art
A bipotential type electron gun is widely used for color picture tubes. The bipotential type electron gun has good high tension characteristics, and a good focussing characteristic as long as it is used at a low beam current. However, when it is used at a high beam current for reproducing a picture of high brightness, a considerable deterioration of resolution is caused due to an excessive enlargement of beam spot which is called "blooming".
The above-mentioned is elucidated with reference to attached FIG. 1, which is a schematic sectional view along the axis of a bipotential type electron gun of prior art. Thermal electrons emitted from the cathode 1 undergo a converging action of an electrostatic lens 4 called cathode lens which is constituted of the cathode 1, a first grid (G1) as a control grid 2 and a second grid (G2) as an acceleration electrode 3. Accordingly, the electrons cross the axis of the electron gun to produce a crossover 5, and then the electrons travel diverging therefrom. The electrons are then preliminarily focussed by a pre-focus lens 7 produced between the second grid (G2) 3 and a third grid (G3) 6 as a focussing grid. Then, the pre-focussed electron beam is led to a main lens 9 constituted with the third grid (G3) 6 and a fourth grid (G4) 8 as a final acceleration grid. The main lens 9 produces a beam spot 12, which is a virtual image 10 of the crossover 5 made by the pre-focus lens, on a fluorescent screen.
It is well-known in electron gun design that in cases where diameter .phi. of electron beam 12' at the main lens 9 is either too small or too large, the diameter of the beam spot 12 becomes large. Accordingly, it is an important matter to control the beam divergence angle a' by pre-focus lens 7 thereby to control the electron beam diameter .phi. to an appropriate value.
In order to obtain a beam spot 12 of a small diameter, the diameter of the virtual image of the crossover 10 must be small; but this becomes more difficult as the beam current increases. In a bipotential type electron gun the potential of the third electron (G3) is only about 10 KV, and therefore the virtual image of the crossover is 1ikely to become large as the beam current increases, thereby increasing the diameter of the beam spot 12.
Relation between the pre-focus lens 7 and the virtual image 10 of the crossover is shown in FIG. 2, wherein curves 13a and 13b show paths of electrons from the central part of the cathode 1, and curves 14a and 14b show paths of the electrons from the peripheral region of the cathode. The above-mentioned pre-focus lens 7 comprises a convergence lens part 7a formed at the outlet part of the second grid (G2) 3 and a divergence lens 7b formed at the inlet part of the third electrode (G3) 6.
Thermal electrons emitted from the central part of the cathode do not undergo much effect from the cathode lens 4, and produce a crossover 5a at a point which is more distant from the face of the cathode 1. This crossover 5a is located in the convex lens 7a, and therefore the electron beams emitted from the central part of the cathode are not subject as much to a converging action of the convex lens 7a, and thereafter are subject to diverging at the concave lens part 7b. Therefore, the electron beams emitted from the central part of the cathode do not substantially receive influence of the pre-focus lens 7.
On the other hand, thermal electrons emitted from the relatively peripheral region of the face of the cathode 1 is greatly influenced by spherical aberration of the cathode lens 4, to produce a crossover 5b at a part nearer to the surface of the cathode 1. The crossover 5b is located at a position before entering the convex lens 7a, and coming into the convex lens 7a with a relatively large diverging angle a. After converged by the convex lens 7a, the electron beams are made slightly divergent by the concave lens 7b, thereby coming in the third grid (G3) 6 with a divergence angle a' and thereafter comes into the main lens 9.
Diameter of the virtual image 10 of the crossover is determined graphically by drawing a set of straight lines 13a' and 13b', which are extended leftward from the straight line part of the electron paths 13a and 13b, and another set of straight lines 14a' and 14b', which are also extended leftward from the straight line part of the electron paths 14a and 14b. The distance between the crossing positions of the above two sets of the straight lines gives the diameter of the virtual image 10 of the crossover. The diameter of the virtual image 10 becomes larger as spherical aberrations of the cathode lens 4 and pre-focus lens 7 become larger.
Generally speaking, lens action of an electron lens formed by an axially symmetrical electric field is given by the following equation (1): ##EQU1## Wherein V is potential on the axis of electron gun,
Z is distance on the axis from the cathode face, PA1 a is axial position at the inlet position of the lens, PA1 b is axial position of the outlet of the lens, and PA1 V.sub.b is the axial potential at the lens outlet position. PA1 Z.sub.1 and Z.sub.2 are the distance on the axis of the electron gun from electron beam emitting face of the cathode to points of the maximum value and a minimum value of the second derivative, respectively, and PA1 D.sub.1 is the diameter of electron passing aperture of the first grid (G1) .
V" is the second derivative of the axial potential V, that is V"=(d.sup.2 V)/(dZ.sup.2),
FIG. 3 shows the axial potential V and its second derivative V" as a function of axial distance Z, and a lower peak 15 corresponds to the part of the cathode lens 4, a higher peak 16 and a valley 17 correspond to the region of the pre-focus lens 7. The positive maximum 16 of the curve of the second derivative V" lies at the outlet part of the second grid (G2) 3, i.e., at the position Z.sub.1, and has the minimum (negative peak) 17 at the part of the inlet part of the third electrode (G3) 6. The lens action is determined by the integration of V"/.sqroot.V, and accordingly, the lower the axial potential V is, the stronger the lens action. The pre-focus lens 7 as a whole functions as a convex lens.
Generally speaking, the spherical aberration of an electron lens is smaller when its aperture is larger and change of electric field forming the electron lens is more gradual. Accordingly, in the prior arts, the electron beam passing apertures of the second grid (G2) 3 and the third grid (G3) 6 were designed as large as possible, and distance between the second grid (G2) and the third grid (G3) were deermined to be as large as possible to produce a moderate electric field distribution. That is, in the prior arts, the distance between the position Z.sub.1 of the maximum potential and Z.sub.2 of the minimum potential were determined to be more than 1.5 D.sub.1 where D.sub.1 is the electron beam passing aperture of the first grid (G1), and the electron beam passing aperture of the third grid (G3) was selected to have a diameter more than twice that of the electron passing aperture of the second grid (G2). In addition and the first derivative of axial potential was kept less than 5.times.10.sup.4 V/cm.